In complementary metal oxide semiconductor (CMOS) integrated circuits, power consumption is related to power supply voltage and operation frequency. In order to reduce power consumption, modern CMOS microprocessors have adopted dynamic power management using performance states. A performance state is typically a power supply voltage and clock frequency combination. An operating system (OS) determines the clock frequency to complete the current tasks and causes an on-chip system management unit to set the clock frequency accordingly. For example, if on average the microprocessor is heavily utilized, then the OS determines that the clock frequency should be increased. On the other hand if on average the microprocessor is lightly utilized, then the OS determines that the clock frequency should be decreased.
Computer systems often run application programs that require real-time execution. For correct execution of these programs, the computer system is expected to meet strict timing deadlines and to complete execution of certain tasks within constrained periods. To meet these time constraints, the OS can increase the performance state as needed. However die temperature increases at higher clock frequencies and power supply voltages, making it harder for the microprocessor to remain within specified thermal limits. Moreover, it is difficult for the integrated circuit to accurately determine how close it is to the thermal limits. Known microprocessors use thermal sensors that check die temperature and reduce the available clock frequency when the die gets too hot. However when the temperature is less than these thermal limits, it is more difficult to estimate how much the power state can be increased without reaching thermal limits. Known microprocessors measure power consumption and assume worst-care ambient temperature to determine how much the power state can be boosted. By assuming worst-case ambient temperature, the microprocessors reduce the amount of available performance increase in more typical environments.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.